A new assay will be used to investigate misincorporation during DNA synthesis. The assay is based on the rate of elongation of a primer strand when DNA synthesis occurs on a natural template in the presence of only 3 of the 4 dNTP's. Conditions favoring misincorporation (e.g. replacement of Mg ions by Mn ions) enhance the rate of primer extension by DNA polymerases when one of the 4 dNTP's is absent. This assay will be used (1) to assess the fidelity of synthesis at different points on the template, (2) to compare the fidelity of different DNA polymerases, (3) to investigate the influence of various auxilliary proteins (DNA unwinding proteins, subunits of E. coli DNA polymerase III holoenzyme, and E. coli recA protein) on the fidelity of DNA synthesis, and (4) to investigate frameshift mutagenesis, which can apparently be detected by the technique described above.